1. Field of the Invention
This invention relates generally to antennas and more particularly to a novel composite antenna structure and method of construction.
2. Description of the Related Art
In general, the function of an antenna is to either radiate or receive electromagnetic energy. The structure of the antenna is dependent on the frequency or wavelength of the electromagnetic energy to be used, and also, in the case of a receiving antenna, on the strength of the signal when it reaches the antenna.
The characteristics of any electromagnetic signal can be described using two parameters. One parameter concerns the frequency or wavelength of the signal. Since frequency and wavelength are reciprocally related, specifying one necessarily infers the other; thus it is common to refer to antennas by the wavelength to be used, since this parameter is useful in determining the physical dimensions of the required antenna. The second parameter is the energy level to be radiated, or the strength of the signal to be received at the antenna.
These two parameters are required to design a suitable antenna. For example, antennas for use with long wavelengths having relatively low frequencies can simply be individual wires having a length of 1/4 to 1/2 the wavelength of the electromagnetic energy. Electromagnetic energy in this region of the electromagnetic spectrum is not rapidly attenuated as it passes through the atmosphere and is also readily reflected by the ionosphere. Thus, signals of this type having relatively low power can be received over relatively great distances.
A disadvantage of signals of this type is that they are unfocused, carry relatively limited amounts of information, and are readily disrupted by atmospheric conditions or solar phenomenon. Thus, certain applications, such as signal transmission by geosynchronous communication satellites, require use of short wavelength, high frequency electromagnetic energy to penetrate the atmosphere and provide for long range communication. Other examples using electromagnetic energy in this range are microwave communication systems and various types of radar.
Electromagnetic energy is transmitted by causing the energy to be radiated from a suitable radiator. By its nature, electromagnetic energy radiates in a multi-directional fashion from a point source. This means that the total signal energy is dispersed in all directions, resulting in a relatively weak signal. This characteristic can be overcome by using extremely large, high power transmitters, radiating on the order of several thousands of watts of energy, as are commonly used for radio or television transmission.
Many applications, however, either require focused, unidirectional transmission patterns, or have structural or weight constraints that prohibit the use of heavy, high power transmitters. For example, most radar systems emit a focused beam of energy that is reflected by a target back to a receiver. The total weight of spacecraft and satellites are limited by the launch capacity of the launch vehicle, and thus cannot use heavy transmitters. Additionally, one result of point source radiation is that the electromagnetic waves diverge from the radiator. Thus, over great distances, this divergence results in a large attenuation of the strength of the signal when it finally encounters a receiver.
To overcome these obstacles, antenna structures have evolved to provide transmission of focused beams of electromagnetic energy. These same structures can also be used to concentrate weak signals to improve reception. One common structure known in the art is the reflecting dish antenna. In a structure of this type, the reflecting dish is shaped, much like a light reflecting mirror, so that it has a focal point. Energy emitted from the focal point is reflected in a concentrated beam; likewise, energy that falls upon the reflector is concentrated at the focal point. Thus, reflecting dish antennas commonly have a transmitter and/or a receiver located at the focal point of the dish.
The dish portion of the antenna can be fashioned from any material, as long as it incorporates a surface that will reflect the electromagnetic energy to be used. Early dish antennas were constructed entirely of metal. However, in applications where signal strength is very low and large reflecting surfaces are required, such structures are very heavy and cannot be used where weight is a factor. Therefore, it is common today to construct dish antennas having a shell fabricated from a rigid, but lightweight material, and then coating the surface with a thin layer of a reflecting metal, such as aluminum.
Another useful characteristic of electromagnetic waves is that they can be polarized. During polarization, the nature of the electromagnetic wave is altered so that the waves oscillate in only one direction, referred to as the polarizing angle. Antennas can be constructed that are sensitive to receiving energy oscillating in only one plane, with the portion of the wave out of the polarizing angle being highly attenuated. A polarizing dish antenna has a reflector that is not continuous, rather, it consists of a plurality of narrow reflective elements whose width and spacing depend on the selected wavelength to be received. This is particularly useful on a spacecraft, since a second lightweight shell, with a polarization grid oriented orthogonally to the grid of the first shell, can be used to transmit or receive a signal of different polarity at the same wavelength without interference. This essentially provides two antennas in the space required for one.
One antenna design frequently used is the parabolic reflecting dish antenna. The parabolic shape can be adjusted to radiate or receive a wide range of frequencies, and its aperture can be shaped to provide a specific radiation pattern. This is particularly useful on an orbiting communication satellite because it allows the antenna designer to tailor the "footprint" of the radiated beam to optimize transmission of the signal to the area of the earth's surface where reception of the signal is desired.
A parabolic dish is essentially a relatively thin walled structure having the shape of a parabola. The dish may be either symmetrical or non-symmetrical about its principle axis. A parabolic dish antenna comprises, essentially, a parabolic reflector and an antenna feed or receiver at the focal point of the reflector. Many different designs and methods of fabrication have been proposed for a variety of applications, ranging from antennas for mobil television relays to complex antennas used by communication satellites.
Parabolic antenna reflectors are commonly manufactured by first forming a core paraboloid having the desired shape. The reflector is then added to the surface of the paraboloid. In a polarizing reflector antenna, the polarizing grid can be a separate piece situated in front of the reflector. This arrangement, however, requires a support structure for the grid, adding unnecessary weight, and precluding the arrangement of two reflectors to form a dual antenna as described above.
The polarizing grid can consist of thin, conductive strips oriented so that they are parallel when viewed along the focal axis of the antenna. The size and spacing of these strips depends upon the frequency of the radiation to be reflected. For example, an antenna designed for use at Ku Band frequencies (approximately 10-14 gigahertz) will have strips that are approximately 0.0003 inches thick, 0.003 inches wide, and spaced 0.02 inches apart.
One technique widely used to construct parabolic reflecting antennas incorporates the polarizing grid into the reflector surface. This polarizing reflecting surface is produced by using an array of narrow strips of a dielectric material cut into specific shapes that allow the strips, while manufactured as a flat sheet, to be configured in three dimensions as a paraboloid. This paraboloid is then adhered to a pre-formed parabolic-shaped core.
The narrow strips, typically 4-8 inches in width, are normally made of a non-conductive plastic such as Kapton (a registered trademark of the DuPont Corporation) and have conductive strips photo-etched from a copper layer plated on the Kapton surface. Since each strip must be unique to conform to the parabolic surface and to ensure that the conductive strips are parallel, the process is expensive and time consuming. One example of such a process is described in U.S. Pat. No. 4,001,836 (Archer et al.).
The requirement of a separate dielectric strip array adds weight to the antenna, and may also affect the thermal expansion coefficient of the antenna. This is particularly disadvantageous for antennas used on communications satellites where total payload weight is a launch constraint and where the antenna will undergo extremes of temperatures as it moves from full sunlight into shadow while orbiting the earth. A parabolic core can be produced from an aramid fiber such as Kevlar (a registered trademark of the DuPont Corporation) having a coefficient of thermal expansion (CTE) of about one part per million per degree Fahrenheit (PPM/F). A low CTE is desirable because thermal distortions of the antenna reflector can limit the useful temperature range in which antenna will function properly. With the present techniques, the addition of the Kapton strips can increase the CTE of the antenna reflector to 2-4 PPM/F. Co-curing the Kapton strips to the Kevlar core lowers the CTE to only 2-3 PPM/F, and adds further complication to the fabrication process. Thus, distortions caused by uneven heating of the antenna will be magnified, resulting in a reduction of receiver sensitivity and degradation of transmission beam patterns.
What has been needed, and heretofore unavailable, is a low cost method of producing a polarizing parabolic dish antenna that has an inherently low CTE with reduced weight and complexity of fabrication. The presently described invention fulfills this need.